The purpose of this investigation was to observe the effect of the excessive administration of cortisone on the mast cells of submaxillary gland of albino rats. The eighteen albino rats weighing about 100gm were divided into five experimental groups (fifteen rats) and one normal group(three rats). Five experimental groups of three rats were treated intramuscularly with 25mg cortisone acetate every 2 hours for 2,4,6,8 and 10 hours. The results were as follows : 1. In the two and four hours-injected rats, the mast cells of the submaxillary gland were similar with those of normal group. 2. In the eight and ten hours-injected rats, the number of the mast cells was markedly decreased and metachromatic granules were tended to be decreased.